The ground vibrational state microwave spectrum of CH 2DI has been studied in the region 26.5-40.0 GHz. From the observation of weak b-type transitions the A0 rotational constant is determined to be 118 905.67 ± 0.04 MHz. The observed a-type and b-type transitions have been used to obtain A, B, C, and all five quartic distortion constants present in the reduced hamiltonian of Watson. The ground vibrational state spectrum of 13CH 2DI has also been observed and analyzed. Observed hyperfine transitions, in both 12CH 2DI and 13CH 2DI, have enabled Xaa, Xbb, Xcc and the angle between the CI bond and the a inertial axis to be determined. The latter is compared with that predicted by the ground state geometry of methyl iodide. The general harmonic force field of methyl iodide has been redetermined using all available data.